As space exploration continues to expand, and ever more space shuttles, rockets, satellites and other aerospace vehicles are launched into orbit, there continues to exist a need for improved safety procedures for dealing with a catastrophic malfunction in a launch vehicle. One area of primary concern during the launching of a space vehicle is that the vehicle maintain its planned course as it exits the atmosphere and enters orbit. If a launch vehicle should get out of control or go off course during the first few minutes after launch, it could result in disastrous consequences depending upon whether and where the vehicle crashes back to earth. In order to protect the public from launch vehicles which veer off course or have been irretrievably damaged during take-off, the National Aeronautics and Space Administration (NASA) has developed control procedures which are known as range safety systems. Through these range safety systems, NASA is able to destroy an out of control launch vehicle before it causes harm to the public.
Traditionally, range safety procedures have involved monitoring the path of a launch vehicle on radar from the moment of launch until it exits the atmosphere, to assure that the vehicle stays within a planned flight range. If the vehicle stays within the desired range, as normally occurs, the vehicle is permitted to continue on its course, whether it be to enter orbit, in the case of a satellite or space shuttle, or reach an intended target, as in the case of a rocket. However, if the vehicle deviates from the intended range, or goes out of control to such an extent that it presents a safety issue to the earth below, then a series of range safety commands may be implemented which culminate in the destruction of the vehicle.
Range safety systems typically include a number of commands, including "Arm", "Destruct", "Safe" and/or "Test" commands, which can be selectively transmitted as radio signals from powerful FM transmitters to a launch vehicle. The Arm and Destruct commands are used to disable and destroy a vehicle respectively, in the event of a malfunction. The Arm command typically turns off the vehicle's rocket motors, while the Destruct command activates a pyrotechnic charge stored on the vehicle in order to destroy the vehicle. In contrast to the Arm and Destruct commands, the Safe command prevents the destruction of the vehicle. A Safe command is typically issued after the vehicle has exited the earth's atmosphere in order to disable the range safety system on the vehicle and prevent either an unintentional or malicious issuance of the Arm and/or Destruct commands.
Even with the most powerful FM transmitters available, it has been difficult if not impossible to effectively transmit range safety command signals to a launch vehicle using only a single transmitter. Therefore, in order to assure that the commands reach a launch vehicle, networks of down range tracking stations have been implemented to relay the command signals to the vehicle. The down range tracking stations in these networks have been spaced out across a large part of the globe in order to assure that the commands can reach the vehicle at each stage of its flight path. Implementing such a complex network of relay stations has required considerable personnel, expense and coordination.
In addition, range safety command signals have traditionally comprised codes made up of a series of eleven dual-tone characters. The specific dual-tone characters for each flight are selected and loaded into the ground transmitter and vehicle just prior to launch, under tight security, in order to limit access to the codes and, thus, the opportunity for sabotage or other undesirable extraneous interference. While the tight security and eleven character length help to reduce the risk of malicious transmission of the command signals, nevertheless a minor risk does exist that a saboteur could decipher the dual-tone codes in sufficient time to destroy or interfere with a launch vehicle. The risk of sabotage could be further reduced by increasing the length of the codes, however, the longer the code, the greater the time required to transmit and detect the code in the vehicle receiver. Therefore, since time can be a critical element in the event a launch vehicle is speeding uncontrollably off course, practical limits must be placed on the code length.
Although range safety commands have been transmitted to launch vehicles exclusively through FM transmitters and down range tracking stations, satellite communication systems have developed as the primary technique for transmitting flight data to the vehicles. The satellite system that has been primarily employed for these data communications is NASA's Tracking and Data Relay Satellite System or TDRSS. With the TDRSS system, flight data is transmitted to the vehicle by spread spectrum with a quadrature phase shift keying (QPSK) modulator and receiver. In this system, the data is spread by one or more pseudorandom noise (PN) pilot sequences and transmitted on the I channel of the QPSK modulator from a ground station to the appropriate TDRSS relay satellite. The relay satellite frequency converts and redirects the data signal to the launch vehicle. A receiver in the launch vehicle locks onto the QPSK carrier signal, and synchronizes a local pilot sequence in the receiver with the incoming pilot sequence in order to despread the flight data.
With certain launch vehicles, a second static pseudorandom noise sequence has been synchronized with the pilot code and transmitted on the Q channel of the QPSK modulator for ranging purposes, such as determining how far the vehicle is from the satellite. However, the use of the Q channel has been limited to this static sequence, and has not been expanded to include the transmission of data.
Accordingly, based upon the above-described complexities associated with present range safety communication systems, it is desirable to have an improved range safety communication system which eliminates the need for down range tracking stations and decreases the time required to transmit commands, while correspondingly increasing the security of the commands. Further, it is desirable to have an improved range safety communication system that is compatible with existing satellite communication systems.